The present invention relates to a fuel supply apparatus for a gas turbine and a control unit for the same adapted to adjust the fuel supplied from a fuel reservoir so as to provide appropriate temperature and pressure before the fuel is sent to a gas turbine combustor.
In general, as shown in FIG. 13, the fuel supply apparatus in practical use for a gas turbine having an output of 100 MW or less is designed to feed such fuel as liquid natural gas (LNG) or other gaseous fuel from a fuel reservoir 1 to a gas turbine combustor 5 via a fuel emergency cutoff valve 2, a gas turbine inlet fuel cutoff valve 3 and a fuel flow regulating valve 4. In the combustor 5, high-pressure air from a compressor 6 which sucks in atmospheric air to generate the high-pressure air is added to the gaseous fuel to produce a combustion gas which is then fed as a working fluid to a gas turbine 7 where it performs expansion work. The rotating torque obtained by the expansion work is used to run a generator 8 to provide electrical output.
Hitherto, in this type of gas turbine in actual use to provide electrical output, the fuel pressure at the time of feeding the fuel from the fuel reservoir 1 to the gas turbine combustor 5 has been given by an expression (1) shown below, taking into account various losses and the pressure balance of the high-pressure air supplied by the compressor 6. EQU P=P1+P2+P3 (1)
where P denotes a fuel pressure required by the gas turbine 7, P1 denotes the discharge pressure of the compressor 6, P2 denotes a pressure loss of the gas turbine combustor 5 including a fuel nozzle, etc., and P3 denotes various losses of a fuel supply piping system.
Typically, a proper fuel pressure required by the gas turbine 7 which is calculated based on the above expression (1) is ranged from 20 to 25 atg.
Although the required fuel pressure values shown above may be appropriate, if any accidental situation should break out, it becomes difficult to generate combustion gas in the gas turbine combustor 5. For this reason, in some cases, a booster 9 is provided at the inlet end of the gas turbine inlet fuel cutoff valve 3 as emergency corrective measures as illustrated in FIG. 14.
In recent years, more power stations are combined with electric power plants which combine a plurality of gas turbine power plants or combine gas turbines with steam turbines in the same premises or in separated premises as shown in FIG. 12 for the purpose of effective use of land. With this trend, the fuel reservoirs for the aggregate power plants are also being integrated. From the viewpoint of an environmental preservation, gaseous fuels such as LNG is frequently used.
When feeding fuel from the single fuel reservoir 1 to power stations A, B, . . . , in order to set the fuel pre-pressure of the gas turbine combustors 5, 5, . . . , of the power stations A, B, . . . , (i.e. the fuel pressure required by the gas turbines) to PA, PB, . . . , an original fuel pressure P0 is determined with the loss of the piping system taken into consideration. The original fuel pressure P0 is adjusted by a pressure regulating valve 10.
The original fuel pressure P0 is decided based on a fuel pre-pressure PD required by the gas turbine combustor 5 of a power station which is located farthest from the fuel reservoir 1, and the fuel pre-pressures PA, PB, . . . , vary depending on the operating situations of the power stations A, B, . . . .
For instance, in an operation mode wherein the power station A is halted, the power station B and a power station C are partially loaded, and a power station D is fully loaded. The fuel pre-pressure of the gas turbine combustors 5, 5, 5, . . . , of the power stations B, C, D, . . . , will be the required value or higher since the supply of fuel to the power station A has been cut off. Hence, there is a demand for reducing the fuel pressure in the operation by providing pressure reducing valves 11, 11, . . . , to obtain the required fuel pre-pressure of the gas turbine combustors 5, 5, 5, . . . , of the power stations B, C, D, . . . .
To adjust the fuel pre-pressure of a gas turbine combustor, the fluctuation in load on the gas turbine can be dealt with satisfactorily by the pressure regulating valve 10 regardless of provision of the booster as long as there is only one combustor. If, however, each gas turbine combustor of a plurality of power stations is required to perform the operation in which the fuel pressure is reduced, then the following several problems are posed.
(1) When reducing the pressure of fuel, excessive reduction of the pressure would cause the saturation temperature of the fuel to drop due to the Joule-Thomson effect, thus accelerating the liquefaction of the fuel. Thus, when the combustion gas is generated in the gas turbine combustor, the rich part of the fuel is unevenly distributed, causing the combustion gas to locally develop an abnormally high temperature. Consequently, a high concentration of NOx may result, the nozzle may be damaged by the local abnormal high temperature at the fuel nozzle portion, or the combustion gas which has locally developed the abnormally high temperature may be generated in the gas turbine 7 and flows through the gas turbine 7, burning the stationary blades or moving blades thereof.
Further, in the case of the fuel rich, since the combustion temperature temporarily increases, it is required to throttle the fuel, and in such case, if a time lag exists, supply pressure in the fuel supply line may be reduced and a back-fire may be caused to the fuel system, which will results in the burning to the upsteam side of the fuel system.
(2) When performing the operation in which the fuel pressure is reduced, the pulsation of the fuel which takes place each time the pressure reducing valve 11 is opened or closed affects the opening or closing of the gas turbine inlet fuel cutoff valve 3 or the fuel flow regulating valve 4. This causes a valve rod to be hunched, leading to a possibility of breaking it. Further, in case of an emergency such as a first cutback operation (isolated operation in a power station), the pressure of the fuel itself considerably changes and there is a danger that the combustion gas in the gas turbine combustor may develop a flameout in some cases. Furthermore, in the case of the fuel rich, since the combustion temperature temporarily increases, there is a feat that a combustor liner, a transition piece, and movable and stationary blades of the gas turbine may be burnt and hence damaged.